Cascode Amplifier Explained: Working, Advantages, and Design Insights

The cascode amplifier is one of the most powerful and commonly used configurations in analog circuit design. It is known for its high gain, high output resistance, and improved bandwidth. The term “cascode” comes from “cascade to cathode” in vacuum tube amplifiers and remains essential in modern MOSFET and BJT-based designs. Understanding how the cascode amplifier works is crucial for analog engineers, as it forms the basis of op-amp input stages, current mirrors, and RF amplifiers.

1. What is a Cascode Amplifier?

A cascode amplifier consists of two transistors connected in series, where the lower device acts as a common-source (or common-emitter) amplifier, and the upper device acts as a common-gate (or common-base) stage. The combination results in high gain and excellent isolation between input and output.

In MOSFET terms, the lower transistor provides voltage gain, while the upper transistor shields it from output voltage variations, keeping its drain voltage nearly constant. This reduces the Miller effect and increases bandwidth.

2. Basic Working Principle

Let’s consider a MOSFET cascode amplifier with transistors M1 and M2:

• M1 (bottom device): Operates as a common-source amplifier. Its drain connects to the source of M2.
• M2 (top device): Operates as a common-gate amplifier. Its gate is biased at a fixed voltage, providing isolation between input and output.

The key idea is that M1’s drain voltage is nearly constant due to M2, so the voltage variation across M1 is minimized. This suppresses channel length modulation, increasing output resistance dramatically.

3. Small-Signal Gain

The small-signal voltage gain of the cascode amplifier is given by:

A_v = -g_m1 (r_o1 || r_o2) ≈ -g_m1 r_o1 r_o2

Since r_o2 is typically large, the overall gain is much higher than a single-transistor amplifier. The combination effectively multiplies the output resistance of the lower transistor by the gain of the upper transistor.

4. High Output Resistance

The total output resistance (R_out) of the cascode amplifier is approximately:

R_out ≈ g_m2 r_o1 r_o2

This means even a moderate g_m2 can amplify the resistance seen at the output, resulting in very high gain and excellent current source behavior. This is why cascodes are used in precision current mirrors and amplifiers.

5. Advantages of Cascode Amplifiers

• Increased Gain: Product of the transconductance and multiplied output resistance results in very high gain.
• High Output Resistance: Ideal for current sources, differential pairs, and mirrors.
• Reduced Miller Effect: Input capacitance is shielded, improving high-frequency response.
• Improved Bandwidth: Because the Miller multiplication of capacitances is minimized.
• Better Isolation: Input and output are isolated, reducing feedback and distortion.

6. Disadvantages and Design Trade-offs

• Reduced Voltage Swing: Stacking two transistors limits the output voltage range, especially in low-voltage designs.
• Biasing Complexity: Proper biasing is needed to keep both transistors in the active region.
• Increased Headroom Requirement: Needs higher supply voltage, which can be problematic in deep submicron technologies.
• Higher Power Consumption: Additional biasing networks may increase current draw.

7. Design Insights for Cascode Implementation

• Ensure the upper transistor’s gate bias maintains both devices in saturation.
• Choose the upper transistor’s W/L ratio to provide high g_m while maintaining low voltage headroom.
• Use longer channel devices for the cascode transistor to improve output resistance.
• In differential amplifiers, cascode loads improve common-mode rejection and gain.

8. Applications of Cascode Amplifiers

• Operational Amplifiers: Cascode stages in op-amp input and gain stages improve gain and bandwidth.
• Current Mirrors: Cascode mirrors achieve near-ideal current copying due to very high output resistance.
• RF Circuits: Used in low-noise amplifiers and mixers to enhance bandwidth and linearity.
• Voltage References: Used for biasing stable current sources in analog circuits.

9. Interview Questions on Cascode Amplifiers

• What is the purpose of a cascode configuration?
• Why does a cascode amplifier have higher output resistance?
• How does cascode configuration improve bandwidth?
• What are the disadvantages of cascode amplifiers in low-voltage design?
• Compare simple amplifier and cascode amplifier performance.

Conclusion

The cascode amplifier offers an elegant solution to the limitations of single-transistor amplifiers by combining high gain, wide bandwidth, and strong isolation. Though it demands careful biasing and more headroom, it remains indispensable in analog design. Whether used in op-amps, current mirrors, or RF front-ends, mastering the cascode concept is essential for any serious analog engineer or interview candidate.